1. Field
The invention relates generally to nuclear power plants and, more particularly, to a method of evaluating the tubes of a steam generator of a nuclear power plant.
2. Description of the Related Art
Nuclear power plants are generally well known. Nuclear power plants can generally be stated as comprising a reactor that includes one or more fuel cells, a primary loop that cools the reactor, and a secondary loop that drives a steam turbine which operates an electrical generator. Such nuclear power plants typically additionally include a heat exchanger between the primary and secondary loops. The heat exchanger typically is in the form of a steam generator which comprises tubes that carry the primary coolant and a plenum that carries the secondary coolant in heat-exchange relationship with the tubes and thus with the primary coolant.
As is also generally known, the tubes of a steam generator are subject to wear from mechanical vibration, corrosion, and other mechanisms. It thus is necessary to periodically inspect the tubes of a steam generator for wear in order to avoid failure of a tube which might result in nuclear contamination of the secondary loop, by way of example. While numerous methodologies have been employed for performing such inspection, such methodologies have not been without limitation.
One method of inspecting the tubes of a steam generator involves the insertion of an eddy current sensor into one or more of the tubes and to receive from the eddy current sensor a signal which typically is in the form of a voltage and a phase angle. An analyst reviewing the signal data typically must possess a high degree of expertise in order to accurately ascertain from the signal data the current condition of the tubes of the steam generator. A typical steam generator might possess between three thousand and twelve thousand tubes, by way of example, with each tube being several hundred inches in length. Thus, the review of eddy current data can require the expenditure of large amounts of time by an analyst. While certain testing protocols may require the testing of fewer than all of the tubes of a steam generator, depending upon the particular protocol, the time in service, and other factors, the analysis of such data still requires significant time and expense.
Among the difficulties involved in the analysis of eddy current data is the determination of whether a signal is indicative of a possible failure of a portion of a tube or whether the signal is not indicative of such a failure. Each tube of a steam generator typically has a number of bends and a number of mechanical supports. In passing an eddy current sensor through such a tube, the signal from the eddy current sensor will vary with each mechanical support and with each bend, and the signal also will vary in the presence of a flaw such as a crack or a dent in the tube. As such, the difficulty in analysis involves the ability to determine whether a change in a signal from an eddy current is indicative of a known geometric aspect of a tube such as a bend or support, in which case further analysis of the signal typically is unnecessary, or whether the change in signal from the eddy current sensor is indicative of a crack or a dent, in which case further analysis of the signal typically is necessary.
Existing methodologies for analyzing tube signals have involved the use of one or more pre-established signal thresholds. However, due to the great variability of tube geometries within a given steam generator and the differing actual condition of each such tube, the use of a limited number of fixed signal thresholds to interpret eddy current signal data from the tubes still results in many portions of many tube signals exceeding the limited number of fixed signal thresholds and therefore requiring further manual examination by an analyst. It thus would be desirable to provide an improved system for assessing a current condition of the tubes of a steam generator.